Shin-ya Koshihara

Magnetic properties of Mn-doped ZnO

We report on the magnetic properties of an oxide-diluted magnetic semiconductor (DMS), Zn0.64Mn0.36O. The temperature dependence of the magnetization shows a spin-glass behavior with the large magnitude of the Curie–Weiss temperature, corresponding to a stronger antiferromagnetic exchange coupling than other II–VI DMSs. The small effective Mn moment (x∼0.02) under high field also represents a strong antiferromagnetic exchange coupling in this compound.

Ferromagnetism in Co-Doped TiO2 Rutile Thin Films Grown by Laser Molecular Beam Epitaxy

Epitaxial TiO2 rutile films were fabricated on α-Al2O3 (1012) substrate in the layer-by-layer fashion by laser molecular beam epitaxy. Ablation with a Co-doped TiO2 target produced single phase of rutile film with the concentration of Co between 0 and 5%. Some ferromagnetic domain structures were observed in CoxTi1-xO2 rutile films by a scanning superconducting quantum interference device microscope at 3 to 90 K. The magnetic hysteresis could also be observed even at room temperature with a magnetic moment of ~ 1 µB/Co atom.

Mapping molecular motions leading to charge delocalization with ultrabright electrons.

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

Effects of Chemically Induced Contraction of a Coordination Polyhedron on the Dynamical Magnetism of Bis(phthalocyaninato)disprosium, a Single-4f-Ionic Single-Molecule Magnet with a Kramers Ground State

Chemically induced longitudinal contraction of the square-antiprism coordination polyhedron of a peripherically substituted bis(phthalocyaninato)dysprosiumate(III), a dysprosium-based single-4f-ionic single-molecule magnet having a J z = +/- (13)/ 2 Kramers doublet ground state, resulted in drastic changes in dynamical magnetism including a doubling of the energy barrier, a 2-order-of-magnitude decrease of the spin reversal rate, a significant rise of the blocking temperature, and the first observation of the emergence of a large remanent magnetization.

Visualizing breathing motion of internal cavities in concert with ligand migration in myoglobin

Proteins harbor a number of cavities of relatively small volume. Although these packing defects are associated with the thermodynamic instability of the proteins, the cavities also play specific roles in controlling protein functions, e.g., ligand migration and binding. This issue has been extensively studied in a well-known protein, myoglobin (Mb). Mb reversibly binds gas ligands at the heme site buried in the protein matrix and possesses several internal cavities in which ligand molecules can reside. It is still an open question as to how a ligand finds its migration pathways between the internal cavities. Here, we report on the dynamic and sequential structural deformation of internal cavities during the ligand migration process in Mb. Our method, the continuous illumination of native carbonmonoxy Mb crystals with pulsed laser at cryogenic temperatures, has revealed that the migration of the CO molecule into each cavity induces structural changes of the amino acid residues around the cavity, which results in the expansion of the cavity with a breathing motion. The sequential motion of the ligand and the cavity suggests a self-opening mechanism of the ligand migration channel arising by induced fit, which is further supported by computational geometry analysis by the Delaunay tessellation method. This result suggests a crucial role of the breathing motion of internal cavities as a general mechanism of ligand migration in a protein matrix.

A gigantic photoinduced dielectric constant of quantum paraelectric perovskite oxides observed under a weak DC electric field

A gigantic enhancement of dielectric constant by UV light irradiation is reported for the first time in the quantum paraelectric family of perovskite oxides, such as SrTiO 3 and KTaO 3 , under a weak DC field. The enhancement has not been observed in ferroelectric SrTi 18 O 3 induced by substitution of 18 O for 16 O. The present result suggests a new class of light-switching phenomena controllable externally.

Developing 100 ps-resolved X-ray structural analysis capabilities on beamline NW14A at the Photon Factory Advanced Ring

NW14A is a newly constructed undulator beamline for 100 ps time-resolved X-ray experiments at the Photon Factory Advanced Ring. This beamline was designed to conduct a wide variety of time-resolved X-ray measurements, such as time-resolved diffraction, scattering and X-ray absorption fine structure. Its versatility is allowed by various instruments, including two undulators, three diffractometers, two pulse laser systems and an X-ray chopper. The potential for the detection of structural changes on the 100 ps time scale at NW14A is demonstrated by two examples of photo-induced structural changes in an organic crystal and photodissociation in solution.

Light-induced ferromagnetism in III-V-based diluted magnetic semiconductor heterostructures

We report for the first time the occurrence of light-induced ferromagnetic order in the III-V-based diluted magnetic semiconductor heterostructures (In,Mn)As/GaSb. We believe that the phenomenon is based on the generation of excess carriers (holes) in the (In,Mn)As layer by the irradiation of light, which enhances a carrier-mediated ferromagnetic interaction between Mn ions.

Direct Probing of Spin State Dynamics Coupled with Electronic and Structural Modifications by Picosecond Time-Resolved XAFS

The first direct observation of the transient spin-state in a disordered magnetic system with time-resolved XAFS is reported. By observing the evolution of the Fe(II) 1s-3d transition, the spin crossover transition from the (1)A(1) low spin state to (5)T(2) high spin state has been directly observed on a picosecond time scale. Moreover, observation of the transient spin state with time-resolved XAFS allows for the investigation of the variations in the electronic state and molecular structure. This unique experimental technique probes the excited states involved in the ultrafast photoinduced reactions in disordered magnetic systems.

Fabrication of a triple tapered probe for near-field optical spectroscopy in UV region based on selective etching of a multistep index fiber

We propose a method to fabricate a novel triple tapered probe for highly sensitive and highly resolved near-field optical microscopy in a near ultraviolet region. The method is based on selective etching of a multistep index fiber. The obtained fiber probe has a triple tapered structure with three taper angles of 60°, 120°, and 60°. With near-field optical microscopy employing the probe, we succeeded in obtaining a photoluminescence image of polysilanes excited by an ultraviolet laser with a wavelength of 351 nm.